This invention relates to a lithium ion secondary cell employing a solid electrolyte.
In the past, a non-aqueous electrolytic solution was generally used as an electrolytic solution for a lithium ion secondary cell. A lithium ion secondary cell employing a polymer electrolyte made of polymer has recently attracted more attention of the industry than such electrolytic solution mainly employing liquid.
The lithium ion secondary cell employing a polymer electrolyte holds a liquid electrolytic solution in the polymer electrolyte and, therefore, has the advantages that there is little possibility of leakage of the liquid, that there is little possibility of corrosion, that short-circuiting between electrodes caused by precipitation of lithium in the form of dendrite can be prevented and that assembly of the cell is easy because the structure of the cell is very simple.
Since lithium ion conductivity of such polymer electrolyte is lower than an electrolyte containing only an electrolytic solution, there has occurred a practice to reduce thickness of the polymer electrolyte. There, however, has arisen a problem in such polymer electrolyte whose thickness is reduced that, since its mechanical strength is reduced, the polymer electrolyte tends to be destroyed resulting in short-circuiting between the positive electrode and the negative electrode.
To solve the above problem and improve mechanical strength of the electrolyte, Japanese Patent Application Laid-open Publication No. 6-140052 proposes a composite electrolyte made by adding an inorganic oxide such as alumina to an electrolyte in the form of gel. The publications also proposes inorganic oxides other than alumina, such as silica and lithium aluminate, as a preferable adder.
If, however, an inorganic oxide such as alumina is added to an electrolyte, there arises the problem that lithium ion conductivity of the composite electrolyte is significantly reduced. Besides, if charging and discharging are performed repeatedly in a lithium ion secondary cell containing this composite electrolyte, reaction takes places between the electrolyte and the inorganic oxide to deteriorate the charging-discharging cycle characteristic of the lithium ion secondary cell.
There have also been proposed various solid electrolyte cells which do not employ an electrolytic solution at all for the sake of safety. Since, however, the positive and negative electrodes and electrolyte composing the cell are all made of solid materials, electro-chemical resistance in the interface between the positive electrode and the electrolyte or between the negative electrode and the electrolyte is very high. For this reason, impedance in the interface is so large that polarization tends to take place. This makes it difficult to realize a cell of a high capacity and a large power.
It is, therefore, an object of the present invention to provide a lithium ion secondary cell which has solved the above problems and has a small interface resistance notwithstanding that a solid electrolyte is employed and, therefore, has a high cell capacity and an excellent charging-discharging cycle characteristic and thereby ensures a stabilized use over a long period of time.